I think you'll find it is because some space is used up when the card is formatted.
It was the same with 2Mb floppy discs, which only had 1.44Mb (or was it 1.38Mb?!!!) of useable space after they were formatted
Are the backgrounds for Aquafish's multifirmware the same as the ones used in wind-ups?
Also, are the ones used in YAFL the same as the ones in wind-ups?
If they are different I will create a gallery on my site for the other types of backgrounds
Cheers